U0038

Code

Generic U — Network/User

Vehicle Communication Bus B Performance

Brand: Generic

Type: U — Network/User

Views: UK: 32 EN: 48 RU: 28

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Page language: EN

Causes

Open, short, or high-resistance wiring on Bus B (CAN/LIN/FlexRay depending on vehicle)
Failed or loose connector/pin (corrosion, bent pin, broken terminal)
Missing or defective termination resistor(s)
Faulty control module attached to Bus B
Electrical noise or interference (aftermarket accessories, damaged shielding)
Low or unstable battery/ignition voltage

Symptoms

MIL (check engine light) or dash warning related to communication
Loss or intermittent operation of modules connected to Bus B (e.g., ABS, BCM, HVAC)
Multiple communication-related DTCs recorded
Intermittent starts or no-crank/no-start if powertrain module communications affected
Diagnostic scan tool shows missing modules or low message frequency on Bus B

What to check

Scan vehicle with a capable scan tool; record all U-codes and module presence on Bus B
Visual inspection of Bus B wiring, connectors, grommets and module harnesses for damage or corrosion
Check for installed aftermarket accessories on Bus B and disconnect for test
Measure DC voltage on both bus lines (CAN_H, CAN_L) with ignition ON and engine OFF
Measure resistance across CAN_H and CAN_L with power OFF — expect ~60 Ω (two 120 Ω terminators in parallel)
Backprobe bus lines with an oscilloscope to verify proper dominant/recessive waveforms and bit timing

Signal parameters

CAN (idle/recessive): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both ~2.5 V when recessive)
CAN (dominant): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V, differential ≈ 2.0 V
Differential voltage (recessive) ≈ 0 V; (dominant) ≈ 2 V
Termination: two 120 Ω resistors (one at each end) producing ~60 Ω across lines with power off
Typical bit rates: 125 kbps, 250 kbps, 500 kbps, or 1 Mbps depending on vehicle
Symptoms on oscilloscope: excessive noise, missing recessive/dominant levels, reflections, or continuous dominant state indicate faults

Diagnostic algorithm

Confirm U0038 present and record freeze/frame data and any related U-codes; clear codes and attempt to re-run self-tests to see if code returns.
Check for other modules reporting lost communication (use scan tool module list). Note which modules are absent or intermittent on Bus B.
Perform a careful visual inspection of all connectors and wiring associated with Bus B modules; look for corrosion, bent pins, broken wires, or evidence of water intrusion.
With ignition ON (engine off), measure DC voltage at CAN_H and CAN_L relative to chassis ground. Compare to expected idle voltages (~2.5 V).
With battery disconnected or ignition off, measure resistance across CAN_H and CAN_L at a convenient connector — expect roughly 60 Ω. If open or high, locate open circuit or missing terminator.
Backprobe bus lines and capture waveforms with an oscilloscope while a scan tool monitors traffic. Look for correct dominant/recessive transitions, excessive noise, reflections, or a constant dominant state.
If a particular module is suspected, disconnect that module (if safe to do so) and observe if the bus behavior and codes change. Reconnect to confirm.
Inspect and test grounds and battery supply to modules on Bus B; repair any poor grounds or unstable supply.
Isolate and repair damaged wiring or connectors: replace corroded terminals, repair shorts/opens, and restore proper shielding/grounding. Replace missing or defective termination resistors.
If wiring and connectors check good and the fault persists, consult manufacturer module diagnostics and consider module replacement or swapping with a known-good unit for verification.
After repairs, clear codes, re-scan, and road test as required to ensure the U0038 does not return and affected systems operate normally.